Antenna arrays provide an efficient way to transmit and receive signals to improve the capacity, data throughput and link range by overcoming undesired channel conditions. Unlike with a single antenna where the parameters are limited to bandwidth, direction and amplitude, the beam pattern for an antenna array can be created by a weight vector which essentially modifies either or both of amplitude and phase of the signal transmitted or received by the antenna. While applying weights can be done by hardware in the radio frequency (RF) domain, it is usually more efficient and practical to apply them in the digital domain.
There are different ways to realize an antenna array, which results in many different technologies, algorithms, classifications, and terminologies. For discussion purposes, we classify them into 2 categories: Multiple-Input-Multiple-Output (MIMO) and Beam forming.
Beam forming is a signal processing technique used in sensor arrays for directional signal transmission or reception. When transmitting, a beam former controls the phase and relative amplitude of the signal at each transmitter, in order to create a desired signal pattern. When receiving, information from different sensors is combined in such a way that the expected pattern of the radiation is recovered. The adjustment is usually done either in the analog domain or in the digital domain via the so-called antenna weight. In the digital domain, weight can be represented by an array of complex numbers. The spatial selectivity is achieved by using adaptive or fixed receive/transmit beam patterns. There are significant performance improvements, or gain, of a fixed beam former compared with an omni-directional antenna. However, it is the adaptive beam forming that has promised much bigger performance improvements in wireless systems. Adaptive beam forming systems usually require certain feedback of the channel information from the device at the other end of the communication system or estimates of the information in order to determine how to adjust the antenna weights.
MIMO is the use of multiple antennas at both the transmitter and receiver to improve communication performance. In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of multiple paths between multiple transmit antennas at the transmitter and multiple receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information.
SISO (single-in-single-out), SIMO (single-in-multiple-out), and MISO (multiple-in-single-out) are the special case of MIMO. Also there are open-loop MIMO and closed-loop MIMO. One example of open-loop MISO is transmit diversity, for example, Space-Time Transmit Diversity (STTD) used in Universal Mobile Telecommunications Systems (UMTS) standard and cdma2000 standard. An example of open-loop SIMO is receiver diversity with MRC combining or switched receiver diversity. MIMO sometimes can be divided into three main categories, spatial multiplexing, transmit diversity or space-time coding, and pre-coding.
Pre-coding can be viewed as a generalized beam forming that can support single-layer or multi-layer transmission in MIMO systems. In the absence of scattering, beam forming results in a well-defined directional pattern, but in general, cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beam forming cannot simultaneously maximize the signal level at all of the receive antennas and pre-coding is used.
A pre-coding scheme can be used in a MIMO system where the antenna configuration is pre-defined. In this case, a codebook that defines the weights can be derived based on the desired directional beam patterns. This type of pre-coding is very much overlapped with the adaptive beam forming based on estimation. In the detailed description, we do not distinguish this from adaptive beam forming based on estimation. Pre-coding can also be used in a MIMO system where the antenna configuration is not pre-determined. In that case, the codebook is derived based on criteria such as maximizing the signal level under certain channel conditions. Note that pre-coding requires knowledge of the channel state information (CSI) at the transmitter. Pre-coding has been used in 4G wireless standards such as Long Term Evolution (LTE).
Placement of the antenna in an antenna arrays is very important. For beam forming, the antenna spacing in a uniform linear array (ULA) should be around λ/2 while for MIMO, the preferable antenna interval is around 4λ to 10λ. A uniform circular array (UCA) is another way to place the antenna. There are also other ways to place an antenna array, such as a 2-D array, etc.
There has been work on coordinated antenna arrays. However, the prior art usually addressed how an individual BTS uses the method to improve the communications with multiple terminals. For example, U.S. patent application 20080075033 described a beam-forming system comprising a cooperative array of wireless terminals coupled to at least one wireless wide area network and communicatively coupled to a wireless local area network configured to provide information exchanges between the wireless terminals. A cooperative beam-forming system uses an antenna array formed by a group of wireless terminals in order to provide antenna-array processing benefits (such as frequency reuse, interference rejection, array-processing gain, and antenna-switching diversity) to the individual wireless terminals. A network access operator facilitates network control functionality between the WWAN and the cooperative array of wireless terminals. This patent application does not address the issue of coordination of multiple base stations each employs a beam forming systems.